The present invention relates to devices for storing data on magnetic tape, and more particularly, to a magnetic tape drive of the streaming type which electronically simulates conventional fast acceleration start/stop tape drives.
Conventional fast acceleration start/stop magnetic tape drives use high acceleration capstan motors to allow the tape to achieve the required operating speed within the minimum inter-record gap. Such motors must have a sophisticated low inertia construction to enable them to achieve high rates of acceleration. Therefore they are expensive and fragile. Typically, vacuum columns are employed as a mechanical buffer in such tape drives to decouple the high inertia load of the supply and take-up reels from the capstan drive.
The growing demand for backup of disk storage has led to the recent introduction of streaming tape drives, such as the F880 MICROSTREAMER (Trademark) tape drive manufactured by Cipher Data Products, Inc. and disclosed in U.S. Pat. No. 4,243,186. In a streaming tape drive, the tape runs more or less continuously, and the capability for fast acceleration is not provided. Therefore, the capstan drive mechanism and vacuum columns are eliminated. Furthermore, streaming tape drives utilize less expensive reel motors than fast acceleration start/stop tape drives. Accordingly, streaming tape drives are generally more reliable and less expensive than fast acceleration start/stop tape drives. While streaming tape drives perform well in a steady state environment involving long continuous write or read operations, they are not suited for applications which require frequent dynamic transtions such as read forward-to-read reverse, or applications requiring inter-record computer processing delays. Such delays can occur where the host computer is disposing of data just acquired from a previous record that has been read and is not in a position to recover further data. Conversely, such delays can also occur where the host computer has experienced delays in acquiring data from a source other than the tape drive and therefore cannot continue a tape write operation.
In the past, buffers have been used to equalize the transfer rates of interfaces such as the RS232C and IEEE488, with the data requirements of a synchronous fast acceleration start/stop tape drive. The aforementioned interfaces may be connected to a host such as a computer or a smart CRT. Such conventional buffers assemble an entire data record from the host before it is written onto tape, and read an entire record from the tape before it is transmitted to the host. In some cases, dual buffers have been utilized to permit one data record to be assembled in one of the buffers while a data record in the other buffer is being transferred.
If a conventional buffer were to be coupled between a host and a streaming tape drive, the buffer would accommodate the different data transfer rates in a steady state mode such as a continuous write operation. However, such a system would be unsatisfactory in an environment characterized by frequent dynamic transitions such as a read forward-to-read reverse operation. This is because a streaming tape drive is not capable of fast accelerations, and therefore rapid bi-directional tape access would not be possible.